This invention relates to a flexible multilayer tubing (commonly referred to as “pipe”) having an outer layer or outer sheath of crosslinked polyethylene (the recognized abbreviation for which is “PEX”) and an inner layer or tubular core of high density polyethylene (“HDPE”), wherein polymer in the core has a substantially higher density than polymer in the outer sheath. Polyethylene (“PE”) is generally regarded as being “high density polyethylene” or “HDPE” when its density is at least 0.941 g/cc (see Encyclopedia of Chemical Technology by Kirk & Othmer, Vol. 17, pg 704, 1996). Because the wall of the multiplayer pipe is predominantly PEX, the multiplayer pipe is referred to as “multilayer PEX pipe”.
Plastic tubing denotes a particular diameter schedule of plastic pipe in which the outside diameter of the tubing is equal to the nominal size plus 3.175 mm or 0.125″ (inch). Plastic pipe outside diameter schedule conforms to ANSI B 36.10. For convenience, and in deference to common usage, plastic tubing having a nominal diameter in the range from 7 mm to 152 mm is referred to hereinafter as “pipe”.
It is well known in the art to subject polyethylene to a variety of crosslinking processes to produce PEX. Such crosslinking processes include addition of peroxide, addition of AZO compounds, electron beam irradiation, and addition of silane, each of which known to enhance certain physical and chemical properties of the polyethylene. In particular, crosslinking has been shown to increase maximum useful temperature, reduce creep, improve chemical resistance, increase abrasion resistance, improve memory characteristics, improve impact resistance, and improve environmental stress crack resistance compared to uncrosslinked polyethylene. For example, U.S. Pat. No. 4,117,195 discloses a method for producing PEX pipe using silane grafted PEX; U.S. Pat. No. 5,756,023 discloses several methods for producing PEX; and U.S. Pat. No. 6,284,178 discloses a method for making PEX having a low enough methanol extraction value (using the ANSI/NSF 61 standard), so as to qualify for use in potable water systems.
It is well recognized that PEX needs to be protected from oxidative degradation but it is also well known that chlorine and hypochlorous acid (HOCl) are just as detrimental to PEX pipe as oxidizing agents in the atmosphere, if not more so. To protect PEX against atmospheric degradants, antioxidants are added to the PEX. Little effort has been directed towards protecting PEX piping in water distribution systems in which degradation occurs not only from the oxygen in the atmosphere but also from chlorine and HOCl in the water migrating from the water into the pipe.
To provide protection in a water distribution system, a multilayer pipe having PEX as the core and an oxygen barrier layer outside the PEX layer is disclosed in PCT publication WO 99/49254; to overcome the mechanical limitations of PEX pipe is adhesively bonded to an outer layer of poly(ethylene-co-vinyl alcohol) (“EVOH”) with another layer of adhesive which is stated to prevent cracking of the pipe, which would otherwise occur, when the pipe was expanded. No details are provided sufficient to enable one to determine the effectiveness of the combination without an undue amount of experimentation. EVOH is known to be an oxygen diffusion-resistant material, but is hydrolyzed in water and susceptible to degradation by chlorine and hypochlorous acid.
U.S. Pat. No. 4,614,208 discloses a multilayer pipe having PEX as the core and an intermediate layer of (“EVOH”), which is covered with an outer layer of impact resistant polyethylene.
If one was to recognize the importance of protection from within, it is expected that one could adhesively bond a tubular (or annular) core of a non-hydrolyzable polymer to the inner surface of PEX pipe, thus forming a barrier layer protecting the PEX against degradation from both chlorine and hypochlorous acid. But, there is no suggestion in the art which polymer provides such properties in a thin cross-section, in the range from as thin as 0.025 mm (1 mil) for 7 mm (0.25″) nominal diameter pipe, to 1.52 mm (0.06″) thick for 152 mm (6″) nominal diameter pipe. Nor is there any suggestion that the polymer chosen be co-extrudable in that thickness under substantially the same extrusion conditions as PEX.
From the foregoing, it will be evident that the problem of coping with degradation of PEX pipe is addressed in diverse ways, few of which are focused on the detrimental long term effects of chlorine deliberately added to water. The effectiveness of HDPE was surprising because it is susceptible to degradation by chlorine and HOCl in water at elevated temperature above about 80° C., and elevated pressure above about 274 kPa (25 psig) over a long period of time more than 20 years; it is also to be expected that the higher crystallinity of HDPE would make it far more resistant to oxidation than PEX. Additionally, it is equally well known that the hoop stress of HDPE at 80° C. declines rapidly as a function of time; there is a visible decline after only 10 hr; the decline accelerates after 100 hr, and at the end of 1000 hr the hoop stress at failure for HDPE is only 2 MN/m2 after having started out with a hoop stress of close to 8 MN/m2 (see “Novel Crosslinking Method for Polyethylene” by H. G. Scott and J. F. Humphries, pgs. 82–85, Modern Plastics, March 1973). Viewing this 4-fold decrease in hoop stress at 80° C. one would not likely consider the use of HDPE in combination with PEX.
In sharp contrast, the hoop stress of PEX at 80° C. declines slowly as a function of time; at the end of 1000 hr the hoop stress at failure for PEX is 7 MN/m2 after having started out with a hoop stress of about 11 MN/m2 (see “Novel Crosslinking Method for Polyethylene” by H. G. Scott and J. F. Humphries, pgs. 82–85, Modern Plastics, March 1973). Because typical hot water piping systems are designed for operation at 80° C., there is even more reason to be concerned with the decrease in hoop stress which would be contributed by the HDPE.